The present invention relates generally to the regeneration of particulate solid sorbents used for desulfurizing hot gases produced by the gasification of carbonaceous fuels, and more particularly to the method of regenerating such sorbents which have sulfating tendencies whereby the formation of sulfates during sorbent regeneration is substantially reduced.
During the gasification of carbonaceous fuels which contain sulfur, undesirable gaseous sulfur bearing compounds such as hydrogen sulfide (H.sub.2 S), carbonyl sulfide (COS), and carbon disulfide (CS.sub.2) are produced. These corrosive and environmentally polluting sulfur-containing compounds have been successfully removed from hot product gas streams generated during the gasification of carbonaceous fuels by employing solid sorbents of metal oxides or mixed metal oxides such as copper oxides, iron oxides, and those containing zinc, especially in the form of zinc ferrite (ZnFe.sub.2 O.sub.4) or zinc titanate (Zn.sub.2 TiO.sub.4, ZnTiO.sub.3, and Zn.sub.2 Ti.sub.3 O.sub.8). Of these solid sorbents, the zinc containing sorbents have been found to be particularly satisfactory for use as hot gas desulfurization sorbents. Zinc-containing sorbents readily absorb the gaseous sulfur compounds from the hot fuel gas to provide a hot fuel gas stream essentially sulfur-free for use in applications such as electrical generating systems using gas turbines, fuel cells, process heating, or in the production of synthetic natural gas or chemicals.
Solid sorbents containing sulfur compounds removed from the sulfur-bearing hot fuel gas, have been essentially fully regenerated for reuse in an absorber, such as a fluidized bed absorber, for removing sulfur from fuel gas containing sulfur species. The regeneration of the sulfided zinc-containing sorbents is readily achieved in a regenerator vessel separate from the absorber by removing sulfided sorbents from the absorber and contacting the sorbents with air in the regenerator at an elevated temperature of at least about 800.degree. F. so as to initiate an exothermic reaction between the oxygen and the sulfur species contained on the sorbents. During this exothermic reaction the temperature of the sorbent increases to a maximum temperature of about 1500.degree. F. so as to convert sulfur species to sulfur oxides (SO.sub.2, SO.sub.3) in the sorbent regeneration gases. These sorbent regeneration gases are removed from the regenerator and disposed of or used in a suitable chemical process such as the formation of elemental sulfur. The solid sorbents are usually repeatedly cycled through the absorber and the regenerator.
In spite of the success of zinc-containing sorbents in the removal of virtually all of the sulfur species from hot fuel gas, it has been found that there is a tendency for many of the present day zinc-containing sorbents to form sulfates, primarily zinc sulfate (ZnSO.sub.4), during the regeneration of the hot gas desulfurization sorbents. The formation of such sulfates is favored by higher concentrations of oxygen and sulfur oxides in the sorbent regeneration gases, and by higher pressures and lower temperatures in the regenerator. Sulfate formation is a result of undesirable side reactions occurring during the regeneration of the sorbent and has a considerable impact on the efficiency of the system. For example, the additional air required for sorbent regeneration due to sulfate formation introduces an efficiency penalty in the regenerator. Also, with sulfate present in the regenerated sorbent, relatively high exothermic sulfate reduction reactions occur in the absorber so as to detract from the efficiency of the system and cause considerable reduction in the heating value of the fuel gas. Further, there is a definite possibility that the structural integrity of the sorbents will be compromised either by sulfate-induced spalling during regeneration or by overheating and sintering of the particulate sorbents which may occur during sulfate reduction reactions in the absorber. Sulfate formation in sorbent regeneration processes using fluidized-bed regeneration also increases the required sorbent circulation rates between the absorber and the regenerator, which provides greater opportunity for attrition of sorbent particulates to occur. Such consequences adversely impact operating costs and the system efficiency by increasing sorbent make-up required for the system and also by increasing the energy required for circulating the sorbent.